A digital image processing method has been proposed, wherein a sample of tone values from the informational portion of the digital image is normalized to produce a tone reproduction function. The tone reproduction function is then applied to the digital image to produce a processed digital image having an aesthetically improved appearance. This approach to tone reproduction is described in the article entitled "A New Approach to Programming in Photomechanical Reproduction" by Yu. Ovchinnikov et al., the 12th IARIGAI Conference Proceedings, Versailles, France, Editor: W. Banks IPC Science and Technology Press, Guildford, England 1974, pages 160-163. Briefly, the method of Ochinnikov et al., involves scanning an original image and randomly sampling the tone values (lightness) occuring in parts of the image where the first derivative of the lightness is above some predetermined minimum threshold value. These sampled tone values are compiled in a relative frequency histogram and the histogram is normalized to produce a tone reproduction function. The tone reproduction function is then applied to the entire digital image to improve the aesthetic appearance of the image.
Improvements to the digital image processing method of Ochinnikov et al., whereby the sample of tone values is selected from an image-dependent "floating" contrast interval selected as a function of the statistics of the tone values in the contrast interval, and wherein the contrast of the image is adjusted as a function of the standard deviation of tone values in the image are disclosed in copending U.S. patent application Ser. Nos. 730,630 and 730,629 respectively. The method is extended to color image processing in the copending U.S. patent application Ser. No. 730,627.
This method produces an aesthetic improvement in the processed image in a majority of instances. However, when the digital image was derived from an over or under exposed photographic original, further improvement in the appearance of the processed image is desirable.
It is the object of the present invention to provide an improved digital image processing method of the type described above for detecting and improving the appearance of a digital image derived from an over or under exposed photographic original.